7-1 Last time □ Wireless link-layer ♦ Introduction • Wireless hosts, base stations, wireless links ♦ Characteristics of wireless links • Signal strength, interference, multipath propagation • Hidden terminal, signal fading problems ♦ 802.11 wireless LANs • CSMA/CA • Frame structure ♦ 802.15 networking ♦ Cellular Internet access
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7-1 Last time □ Wireless link-layer ♦ Introduction Wireless hosts, base stations, wireless links ♦ Characteristics of wireless links Signal strength, interference,
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Transcript
7-1
Last time□ Wireless link-layer
♦ Introduction• Wireless hosts, base stations, wireless links
♦ Characteristics of wireless links• Signal strength, interference, multipath propagation• Hidden terminal, signal fading problems
services:♦ network layer service models♦ forwarding versus routing♦ how a router works♦ routing (path selection)♦ dealing with scale♦ advanced topics: IPv6, mobility
□ Instantiation, implementation in the Internet
7-4
Chapter 4: Network Layer
□ 4. 1 Introduction□ 4.2 Virtual circuit and
datagram networks□ 4.3 What’s inside a
router□ 4.4 IP: Internet Protocol
♦ Datagram format♦ IPv4 addressing♦ ICMP♦ IPv6
□ 4.5 Routing algorithms♦ Link state♦ Distance Vector♦ Hierarchical routing
□ 4.6 Routing in the Internet♦ RIP♦ OSPF♦ BGP
□ 4.7 Broadcast and multicast routing
7-5
Network layer□ Move segment from
sending to receiving host □ On sending side
encapsulates segments into datagrams
□ On receiving side, delivers segments to transport layer
□ Network layer protocols in every host, router♦ But not switches, hubs
□ Router examines header fields in all IP datagrams passing through it
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
7-6
Two Key Network-Layer Functions
□ Forwarding: move packets from router’s input to appropriate router output
□ Routing: determine route taken by packets from source to dest.
♦ routing algorithms
analogy:
□ Routing: process of planning trip from source to dest
□ Forwarding: process of getting through single interchange
7-7
1
23
0111
value in arrivingpacket’s header
routing algorithm
local forwarding tableheader value output link
0100010101111001
3221
Interplay between routing and forwarding
7-8
Connection setup
□ 3rd important function in some network architectures:♦ ATM, frame relay, X.25
□ Before datagrams flow, two end hosts and intervening routers establish virtual connection♦ routers get involved
7-9
Network service model
Q: What service model for “channel” transporting packets from sender to receiver?
Example services for individual packets:
□ guaranteed delivery□ guaranteed delivery
with less than 40 msec delay
Example services for a flow of packets:
□ in-order packet delivery□ guaranteed minimum
bandwidth to flow□ restrictions on changes
in inter-packet spacing
7-10
Chapter 4: Network Layer
□ 4. 1 Introduction□ 4.2 Virtual circuit and
datagram networks□ 4.3 What’s inside a
router□ 4.4 IP: Internet Protocol
♦ Datagram format♦ IPv4 addressing♦ ICMP♦ IPv6
□ 4.5 Routing algorithms♦ Link state♦ Distance Vector♦ Hierarchical routing
□ 4.6 Routing in the Internet♦ RIP♦ OSPF♦ BGP
□ 4.7 Broadcast and multicast routing
7-11
Network layer connection and connectionless service
□ Datagram network provides network-layer connectionless service
□ VC network provides network-layer connection service
□ Specifically:♦ service: host-to-host♦ no choice: network provides one or the other♦ implementation: in network core
7-12
Virtual circuits
□ Call setup, teardown for each call before data can flow
□ Each packet carries VC identifier (not destination host address)
□ Every router on source-dest path maintains “state” for each passing connection
□ Link, router resources (bandwidth, buffers) may be allocated to VC (dedicated resources = predictable service)
Source-to-dest path behaves much like telephone circuit♦ performance-wise♦ network actions along source-to-dest path
7-13
VC implementation
A VC consists of:1. path from source to destination2. VC numbers, one number for each link along
path3. entries in forwarding tables in routers along path
□ packet belonging to VC carries VC number (rather than dest address)
□ VC number can be changed on each link.♦ New VC number comes from forwarding table